I'm using Acer T310(with SiS661FX), I've updated the BIOS provided by here, and also the newest driver
I've bought a new 20" monitor with resolution 1600x900
for 1440x900 it works perfectly

but 1600x900 it works completely strange, and the OSD of the monitor said that the input signal was 1600x1200 instead of 1600x900
But actually when I changed the computer to 1600x1200 output, the same problem occured.